1. Field of the Invention
This invention relates to a method and apparatus for compensating a control system to eliminate the undesirable hysteresis which causes an error in the actuation of a controlled device in response to a control signal.
2. Description of the Prior Art
Hysteresis can cause erratic action in a control system in that the amplitude of the output of the system is not directly related to the amplitude of the input. In control systems such as in turbine engines where the output drives a mechanical device, there can be a substantial error in the response of the controlled device in response to a request signal.
Complete removal of hysteresis from a control system is not usually possible as a result of the many different elements each of which contribute hysteresis within the control system. One method of compensating for hysteresis used in the prior art is the application of a high frequency, low amplitude signal which is superimposed on the input signal to mask the hysteresis dead zone. This prior art technique has a disadvantage in that the duty cycle of the control system increases together with accompanying output fluctuations.
A particular problem with control systems having hysteresis is the fact that the output or controlled device does not immediately follow the commands of the input signal whenever the input changes direction. A dead zone occurs each time that there is a reversal in the direction of the input signal.
The present invention overcomes the disadvantages of the prior art and provides a novel method and apparatus for overcoming the hysteresis dead zone in the control systems. Whenever the input changes direction, a bias is applied to the input in a direction such that the hysteresis dead zone is overriden. As a result, the amplitude of the output is directly related to the amplitude of the input.